Urban ReLeaf is an action-oriented mission to advance citizen-powered science as a central resource for inclusive urban green planning in support of local policy making, European strategies and monitoring systems (e.g., the Green Deal, Copernicus), and global efforts such as the SDGs (e.g., Target 11.7) and GEOSS. The project will: (i) assess current urban greening policy processes within six European cities and co-create solutions that use citizen observations to complement existing data ecosystems and decision-making; (ii) support the validation and long-term inclusion of active and passive data from citizens for urban environmental monitoring within authoritative data streams, including GEOSS & Copernicus; (iii) Mobilize and empower communities through widespread participation in issues of public interest surrounding urban green infrastructure; (iv) develop a community of practice around topics related to the use of citizen observations for urban planning to foster knowledge exchange and develop capacities across multiple sectors; (v) produce flexible and innovative governance solutions to help scale-out inclusive urban green transitions in support of the European Green Deal and UN Sustainable Development Goals; and (iv) promote recognition, adoption and trust of citizen observations and other novel data ecosystems for environmental monitoring to trigger innovation within public institutions. The project is coordinated by IIASA, who led the WeObserve project, and involves partners from previous successful H2020 citizen observatory projects and as well as six cities (Athens, Cascais, Dundee, Mannheim, Riga Utrecht) that are willing to embrace new data streams for urban greening planning and policy making and to fully engage with citizens including marginalized and vulnerable groups. Urban ReLeaf is also supported by technology partners and those with strong links to Copernicus and GEO.